LOCALMAILFILTER(7)					      Double Precision, Inc.						LOCALMAILFILTER(7)

NAME

       localmailfilter - Local mail filtering

SYNOPSIS

       echo /usr/bin/maildrop >/etc/courier/maildropfilter

       mkdir $HOME/.mailfilters

       vi $HOME/.mailfilters/rcptfilter $HOME/.mailfilters/rcptfilter-ext

       vi $HOME/.mailfilters/smtpfilter $HOME/.mailfilters/smtpfilter-ext

       chmod 700 $HOME/.mailfilters

       chmod 600 $HOME/.mailfilters/*

DESCRIPTION

       The maildrop mail filter can be used by the Courier mail server as a mail filtering engine, rejecting unwanted mail on a per-recipient
       basis.

       The actual filtering interface used by the Courier mail server does not really require that maildrop must be used as a mail filtering
       engine, it just so happens that maildrop has a compatible interface that can be used right out of the box. The following brief information
       can be used to craft a homebrewed mail filter to take maildrop's place.

       The local mail filter only works for addresses that correspond to local accounts. This filtering is not used if the recipient is a remote
       address on another mail server. The local mail filter is disabled by default. To enable local mail filtering you will need to initialize
       the /etc/courier/maildropfilter configuration file to contain the pathname to your local mail filter.

       Local mail filtering is performed in two distinct phases:

       Recipient filters
	   When the Courier mail server receives an address naming a local mail recipient, the local mail recipient's mail filter is executed
	   before the Courier mail server acknowledges the address. The local mail filter tells the Courier mail server whether to: A) accept
	   message unconditionally - the message is whitelisted; B) reject the message unconditionally - the Courier mail server tells the other
	   mail server that the recipient address is invalid; or C) accept this recipient, but run the content mail filter, once the message's
	   contents are available.

       Content filters
	   After receiving the contents of the message, the mail filter is executed again for any recipients whose recipient filters used the
	   third option. The content filter can now examine the contents of the message, and indicate whether the message should be accepted or
	   rejected. Content filtering is not available for alias addresses.

       It should be noted that mail filtering is executed as an integral part of receiving a message from a remote mail server. If the message is
       rejected, the Courier mail server refuses to accept the message for delivery.

       The local mail filter will be invoked as follows:

	   HOME=$HOME FILTER -D uid/gid -M filter

       The local mail filter will NOT be invoked as root, so if it needs to access files in the recipient's account, it must be installed setuid
       to root (as maildrop is installed by default).

       "uid/gid" is the recipient account's system userid and group id, respectively. The recipient account's home directory is placed in the HOME
       environment variable, prior to running FILTER, and "filter" is set as follows:

       rcptfilter
	   The mail filter is invoked initially when the remote mail server specifies this address as a recipient.  FILTER should terminate with
	   one of the following exit codes: 0 - this sender is acceptable; 99 - this sender is acceptable, but I want to run the content filter
	   for this the message; any other non-zero exit code - the sender is not acceptable, reject the message.

       smtpfilter
	   If FILTER terminates with exit code 99, FILTER runs again with this parameter set to the word smtpfilter. FILTER will be invoked once
	   the message has been received from the remote mail server, but not yet acknowledged. If FILTER terminates with a non-zero exit code,
	   the message is rejected. If FILTER terminated with the exit code of zero, the message is accepted.

       rcptfilter-ext, smtpfilter-ext
	   If the recipient created sub-addresses - see dot-courier(5)[1] - a dash followed by the subaddress "ext" is appended to the name of the
	   filter.

       rcptfilter-alias-ext
	   This is how FILTER gets invoked if the address is a locally defined mail alias (ext is the alias name).

       The rcptfilter invocation must terminate with a zero exit code when the message originates from a mailing list or any other source that
       should be considered as "whitelisted". This filtering model does not fit very well with some mail transfer protocols, so unless trusted
       sources are explicitly declared to be whitelisted, there is a remote possibility that the recipient will be removed from a mailing list
       because of a poorly-written mail filter from some other recipient of the same message. The 0 return exit code (which is the implied default
       if no mail filtering is installed) protects the recipient from being adversely affected, in any way, by anyone else's mail filter.

       The mail filters may print a diagnostic message before rejecting a message. The diagnostic message will be returned to the sending mail
       relay, where possible.

       The mail filters inherit environment variables that describe the incoming mail. The following environment variables are provided by
       default:

       SENDER
	   The return address on the message.

       TCPREMOTEHOST, TCPREMOTEIP
	   When the message is received via ESMTP, these variables specify the remote IP address and the corresponding hostname. Hostname is empty
	   if the IP address does not have a reverse DNS record, or is set to "softdnserr" if there was a temporary failure while looking up this
	   IP address.

       BLOCK2
	   The default the Courier mail server configuration sets this environment variable if the remote IP address is listed in an unsecured
	   relay blacklist. See /etc/courier/esmtpd for more information. Other environment variables may also be available. For mail received via
	   ESMTP, environment variables are usually set in the /etc/courier/smtpaccess configuration file.

   maildrop implementation
       Maildrop implements this mail filtering API as follows:

       $HOME/.mailfilters
	   This directory contains the filtering recipes. This directory, and its contents, cannot have any group or world permissions.

       smtpfilter*, rcptfilter*
	   These mail filtering recipes directly correspond to the events defined in the previous section. Maildrop's "import" statement can be
	   used to gain access to the environment variables (these mail filters are executed in maildrop's embedded mode). The mail filtering
	   recipes can set the EXITCODE variable appropriately before terminating, in order to accept or reject the message.

       See maildrop(1)[2] for more information.

   Filtering mail to aliases
       The /etc/courier/aliases configuration file is used to mail aliases, see makealiases(8)[3]. The system administrator may set aside a
       reserved local account that will be used to specify a local mail filter for messages addressed to aliases. The configuration file
       /etc/courier/aliasfilteracct specifies the home directory of the mail account that will be used to filter alias recipients.

       For example, if /etc/courier/aliasfilteracct contains /home/admin, then the Courier mail server runs the mail filter as follows:

	   HOME=/home/admin FILTER -D uid/gid -M rcptfilter-alias-name

       Here, "uid/gid" is owner uid and gid of the specified directory NOTE: "name" is a fully qualified address, and the local aliases listed in
       /etc/courier/aliases do not typically include the domain name. If defines an alias called "system", for example, the -M option will
       probably be "system@example.com", if example.com is the contents of /etc/courier/me configuration file.

       Unfortunately, currently it is not possible to specify content filters (a.k.a. smtpfilters) for aliases, only recipient filters.

FILES

       /etc/courier/maildropfilter
	   Local mail filtering engine.

       /etc/courier/aliasfilteracct
	   Account that is used to filter mail to aliases.

SEE ALSO

       courierfilter(8)[4], maildrop(1)[2].

AUTHOR

       Sam Varshavchik
	   Author

NOTES

	1. dot-courier(5)
	   [set $man.base.url.for.relative.links]/dot-courier.html

	2. maildrop(1)
	   [set $man.base.url.for.relative.links]/maildrop.html

	3. makealiases(8)
	   [set $man.base.url.for.relative.links]/makealiases.html

	4. courierfilter(8)
	   [set $man.base.url.for.relative.links]/courierfilter.html

Courier Mail Server						    08/30/2011							LOCALMAILFILTER(7)